libmvec, non-finites and fast-math

[Julian Taylor <jtaylor dot debian at googlemail dot com>]

hi,

with commit 533f9bebf969060e64c66681e275c03d6e49fcc9 the vector math 
functions were changed to call the _finite versions of the scalar 
operations.
While it makes sense in that it equalizes the results for scalar and 
vectorized math for full programs compiled with -ffast-math it hurts 
applications that only want to use that flag in part of their code.
-ffast-math implies a lot optimizations you do in general not want, in 
particular the -ffinite-math flag.

I was using libmvec in a library providing numerical operations to 
applications. To do this and not get bad ffast-math flag infecting the 
whole library I put the relevant math operations on larger data blocks 
into a single file compiled with ffast-math. As the implementation in 
libmvec respects non-finite input this worked out fine. In particular 
power(nan, 0) kept returning 1 as the scalar variant in files without 
-ffast-math.

With this new commit this is broken. The vector variant will return 
different results than the scalar code.
To get around this I now need to check for non-finites manually in my 
fast-math file. But of course compiler now gets in the way as it 
optimizes away the non-finite checks.

It seems the only option I have left is now calling the _ZGV functions 
directly and doing my own vectorized non-finite checks and blend over 
the results. This option is quite appealing as it also removes the GCC 
dependency of the functionality but I am not sure if these functions are 
really to be used by applications/libraries.

So now how to solve this problem. My suggestion would be to relax the 
requirements for vectorizing the math functions to 
-funsafe-math-optimizations instead of -ffast-math, at least when glibc 
or svml are the backends.
Then glibc should to provide _ZGV*_finite variants like it does for the 
scalar functions. That way when the whole program is compiled with 
-ffast-math you get consistent results as all calls will got to the 
_finite variants and when this is not the case all calls go to the 
regular ones.
Thoughts?

cheers,
Julian Taylor